In the wireless communication system adopting OFDM (Orthogonal Frequency Division Multiplexing), if not all of the base stations in a cell are equipped with GPS (Global Positioning System) or Galileo synchronization system, in order to carry out related combination, a synchronization method with an accuracy of microsecond level is required to realize strict synchronization among base stations. If the synchronization among base stations can not satisfy the above-mentioned requirement, a signal arriving at the user equipment receiver may be outside the CP window, which will lead to the inter-symbol interference and affect pilot estimation and data receipt.
Solutions of realizing the aforementioned synchronization in the prior art comprise:
1. A method proposed in 3GPP UTRAN TDD (UMTS Terrestrial Radio Access Network Time Division Duplex), namely the synchronization among a plurality of base stations is realized by using an input synchronization port and an output synchronization port of a base station (also referred to as Node B). As shown in FIG. 1, with its input synchronization port, a base station A receives a synchronization signal sent from an external reference source (e.g. GPS) via the cable between them, so that it can be synchronized with the external reference source; likewise, with the output synchronization port, the base station A sends a synchronization signal to a base station B via the cable between them, then the base station B is indirectly synchronized to the external reference source by way of being synchronized to the base station A. The synchronization of other base stations shown in FIG. 1 can be deduced by analogy.
By using the synchronization method that base stations are connected via cable directly, the base stations to be synchronized are only required to be connected in the form of a chain. All of the base stations on the chain can be synchronized by using only one external time reference source.
The drawback of the solution lies in its weak robustness. If a problem appears in the synchronization of a base station (e.g. the synchronization port can not work properly), then every base station in the downstream of the synchronization chain can not be synchronized with those synchronized base stations;
The weakness of the solution further lies in that synchronization error will be accumulated with the increase of the number of base stains on the chain, namely error accumulation will occur, which makes the variance of the error larger.
2. A synchronization method adopted in WCDMA TDD and TD-SCDMA systems, synchronization among base stations can be realized by transmitting cell synchronization time reference in predefined Physical Random Access Channel (PRACH) or Downstream Pilot Channel (DWPCH) on the air interface.
In this solution, Radio Network Controller (RNC) prearranges transmission time slot. No matter whether the synchronization burst is transmitted in PRACH or DWPCH, at least one base station in a cell cluster is connected with the external time reference source (e.g. GPS), other base stations are then synchronized with the base stations connected with the external time reference source. While controlling the transmission of base station synchronization reference signal and the execution of measurement, RNC is also responsible for sending the time adjustment instruction based on synchronization difference signal reported by base stations. Therefore, RNC is the master controller of the whole synchronization process.
The disadvantage of the solution is that the synchronization process of a base station requires that a base station belonging to an upper layer or a master base station works as a control base station controlling the transmission of time reference signals and the receipt of time slots, so that the transmitter and the receiver coordinate with each other. This is only applicable for TDD mode and can not be realized in FDD (Frequency Division Duplex) mode.
Hence, there is an urgent need to propose a new synchronization solution that can avoid the aforementioned problems at the same time when strict synchronization among base stations is realized.